In the UMTS system, the power of a mobile station (MS) telephone must be tightly controlled so that this level is neither too low nor too high. For example, in a CDMA access system, excess output power of a MS may impede other connections on the same channel.
During a connection, the base station (BS) regularly sends (at a rate of 1.5 kHz) commands to the MS which give a correction to the output power. These commands are defined in 3GPP 25.101 and they are generally communicated at between −3 and +3 dB. The MS is required to effectively and consistently respond with an increase in output power when commanded to output a higher level and vice versa, otherwise the entire channel and all the connections could suffer. Further, step tolerances are specified and the total dynamic range is required to be more than 70 dB.
In the past an open loop concept has generally been used in UMTS telephones; that is, the power was not directly monitored within the telephone hardware.
However, new requirements on tolerances and on effective radiated output power require phone manufacturers to switch to a closed loop approach, in particular, to optimize performance when the antenna is mismatched, while guaranteeing that the specified maximum radiation (SAR) is not overcome. Closed loop systems have long been used in GMSK systems (constant envelope), but their use for UMTS systems is made much more difficult by the presence of wide bandwidth amplitude modulation having a large crest factor as well as requiring that the power stabilize within 50 us.
Accordingly, there is a continued need to improve closed loop power control in UMTS systems.